As known in the prior art, the use of superheated steam as the drying medium is more advantageous than the use of hot air, because in such a case it is also possible to utilize the hot moisture separated from the web. When paper is dried by means of superheated steam, it is known in the prior art to use devices in which the paper web runs on support of a wire and in which, at the side of the paper web, an impingement dryer is placed which includes blow openings for steam and exhaust openings for the return steam. The area of the impingement dryer has been formed as a closed space by means of an enclosure such as a hood. Inside the enclosure, there can also be several separate dryers. The closed space in the interior of the enclosure is usually filled with steam whose temperature is somewhat higher than 100.degree. C. Compared with conventional impingement drying processes provided with an impingement drying hood, an advantage of an enclosed arrangement is that the number of potential leakage points is minimized to two, i.e., the point at which the paper web is passed into an interior of the enclosure and the point at which the paper web is passed out of the interior of the enclosure.
The prior art constructions involve, among other things, the drawback that, with such an enclosure, there should not occur any web breaks or equivalent in the interior of the enclosure, because entering into the interior of the enclosure for repair and/or maintenance operations requires a time-consuming process of emptying the interior of steam and a period of cooling of the equipment and, after the maintenance or cleaning, a long start-up period, during which period the air must be removed and the equipment is heated to the operational temperature, which in itself lowers the capacity of the equipment and causes undue expenses.
In the prior art constructions, the paper web often runs on support of a wire, and the web is kept in contact with the wire by means of a difference in pressure across the wire. The drying air jets of the impingement drying are in themselves insufficient for producing this difference in pressure because the process between the nozzle face and the paper is somewhat dynamic, in which connection the paper web can be separated from the wire.
In the prior art, mechanical seals have been suggested as a solution, which seals permit the maintaining of a certain difference in pressure between the top and bottom portions of the enclosure. However, mechanical seals are not favorable because they drag against the wire and thus wear the wire. Further, the use of mechanical seals has the consequence that the difference in pressure must be maintained over the length of the entire enclosure from the inlet opening of the paper to the outlet opening, because cross-direction additional seals for maintaining a difference in pressure make the equipment considerably more complex. For this reason, at the inlet and outlet openings of the enclosure, there is a pressure above the wire and/or a vacuum below the wire. However, this can result in leakage of steam out of the enclosure above the wire or in flow of air into the enclosure below the wire at the openings if the sealing arrangement of the openings for the web is not fully sealed.
Further, alignment of a mechanical seal is difficult, because the seal must coincide with the edge of the paper web with adequate precision, which is very difficult to accomplish under production conditions. If leakage occurs through the wire outside the edges of the paper web, the result is a reduced difference in pressure or a steam flow from the top side to the bottom side.
Besides sealing, further important factors in applications of impingement drying of a paper web or an equivalent web-like material, from the point of view of runnability, include the keeping or maintenance of the web substantially straight and the constant distance of the web from the blow devices. In order to control the running of the web, it is known to use a suction box placed underneath the wire, but the faces of the suction box that drag against the wire as well as the exhaust suction produced by the suction box cause problems for the runnability of the web and affect its stability on the face of the wire.
With respect to prior art related to the present invention, reference is made to the current assignee's Finnish Patent No. 67,107 (corresponding to U.S. Pat. No. 4,551,203) which describes an arrangement for passing a paper web from the press section into the dryer section, in which, to the side of the drying wire that supports the web, a number of air jets are blown at a speed substantially higher than the speed of the drying wire. The outlet direction of these jets is substantially the same as the running direction of the drying wire at that location, and air is ejected by means of this blowing of air out of the space placed between the drying wire and the wall placed in connection with the members that produce the air jets. In this arrangement, blow boxes are employed, which are positioned on the run of the drying wire at the side of the wire, which extend substantially across the entire width of the web and include one or more nozzle slots. In the blow boxes, the walls placed facing the run of the drying wire are plane (flat) and substantially parallel to the run of the drying wire.
It is known in the prior art to use various impingement-drying/through-drying units for evaporation drying of a paper web. In these units, the paper web is dried so that, by means of the impingement drying devices, hot air or superheated steam is blown in a direction substantially perpendicular to the paper at a relatively high speed. In these constructions, the paper web or equivalent web-like material often runs on support of a wire, roll, cylinder or equivalent support member. With respect to the prior art constructions related to evaporation drying of a paper web and based on impingement drying, reference is made to U.S. Pat. No. 4,361,466, which describes a web drying method and apparatus that employ hot air, and to U.S. Pat. No. 5,210,958, which describes an apparatus and method for drying a paper web in which superheated steam is employed.
Reference is also made to Finnish Patent Application No. 971713 (corresponding to International Application No. PCT/FI 98/00350) which describes a dryer section in which the first drying unit is a planar drying unit including a blow hood under which the web to be dried runs horizontally in a plane supported by a wire. The planar, horizontal run of the wire is enabled by appropriate positioning of grooved rolls and/or suction or blow boxes.